1. Field of the Invention
This invention relates to the field of fiber pressure/force sensors, in particular, to the manufacture of optical fiber pressure sensors using a fiber ringdown apparatus and to a method of determining pressure using such sensors.
2. Background of the Technology
During the past 30 years, fiber optical sensor technology has progressed rapidly, outperforming conventional sensors with sensors having high sensitivity, fast response, low cost, lightweight, and immunity to electromagnetic interference. These improved sensors have been used in a wide range of industries, including aerospace, military, petrochemical, transportation, building and structural monitoring, chemical, and biomedical sectors.
Within the last decade, cavity ringdown spectroscopic (CRDS) techniques have gained rapid development. Ringdown-based instruments are on the market and being used for trace moisture analysis in semiconductor manufacturing industries. Applications of cavity ringdown (CRD) techniques are also expanding into uses in many other areas, such as environmental monitoring and medical diagnostics. The evolution of CRD techniques has led to diversified techniques, which can be classified by configurations of ringdown cavities, such as, for example, the initial mirror-base CRDS, internal reflection-based prism CRDS and fiber end-coated CRDS, and the more recent fiber Bragg grating CRDS.
Within the last two years, a new fiber ringdown technique has emerged. Functionally, this type of fiber ringdown technique resembles the standard high reflectivity CRDS with the exception that it does not require high reflectivity parts such as ring down mirrors. The new fiber ringdown technique utilizes an optical resonator (an optical fiber loop) as the ringdown cavity. In operation, initially light radiation is coupled into the fiber loop. When the light source is rapidly shutoff, the resultant light “rings” inside the fiber loop for many round trips. In each round trip, a small fraction of light leaks into a photodetector through a fiber coupler while the rest of the light continues to ring in the optical fiber loop and experiences internal fiber transmission losses. The signal intensity observed by the detector follows an exponential decay. The lower the losses of the light in the fiber, the longer is the decay time constant (ringdown time).
In 2001, a complicated fiber loop configuration was developed for a direct gas phase absorption measurement. See Stewart, et at., “An investigation of an optical fibre amplifier loop for intra-cavity and ring-down cavity loss measurements”, Measurement Science and Technology, 12:843-849 (2001), the complete disclosure of which is fully incorporated herein by reference. In 2002, Brown et al. reported progress on a fiber loop ringdown technique developed for liquid phase detections using a simplified approach. See Brown, et al., “Fiber-loop ring-down spectroscopy”, Journal of Chemical Physics, 117:10444 (2002), the complete disclosure of which is fully incorporated herein by reference.
U.S. Patent Application Publication No. 2003/0107739, published Jun. 12, 2003 discloses a fiber loop ringdown apparatus for the detection of liquid samples and water vapor, the complete disclosure of which is fully incorporated herein by reference. Thus far all ringdown techniques have been limited to trace gases/liquids spectral measurements. There has been no report or suggestion for the use of the ringdown concept for fiber pressure sensor development.
In recent years, the development of fiber pressure sensors has been primarily based on the foundation of an interference concept, such as, for example, the well known fiber Fabry-Perot interference (FFPI). See, for example, U.S. Patent Publication No. 2003/0138185, published Jul. 24, 2003, the complete disclosure of which is fully incorporated herein by reference. There are many methods that can be used to design and fabricate a FFPI cavity, but the basic principle remains the same. The external changes of pressure or temperature induce the variations of fiber refractive index, or fiber length, or both, resulting in a phase shift of the two coherent light beams changes. The pressure is measured by processing the interference patterns. Similar means are used for temperature measurements. This technique is of high sensitivity, high accuracy, and rapid response. FFPI sensors can be widely used for pressure sensing in a variety of environments. At present, the commercial products of this type of sensor can measure pressure up to 100 psi (69 bars) with a temperature tolerance limit up to 250° C. To date, the reported laboratory measuring limit is 6000 psi and the highest surviving temperature is 800° C. This therefore can be generally considered as very rugged among conventional pressure sensors. However, the FFPI sensors still cannot meet the special needs in aerospace technologies such as propulsion testing, where the upper pressure limit can reach to 15,000 psi. At the same time, the extreme temperatures can be as low as 19 K (34 R).
Another type of newly emerged fiber pressure sensor is based on fiber Bragg gratings (FBGs). In these sensors, a grating, or multiple gratings are written on a small section of fiber. When the wavelength of the light source injected into the fiber satisfies the Bragg condition, the light of this wavelength is strongly reflected while light of other wavelengths is transmitted. If the fiber is exposed to a pressure, which induces the variation of physical parameters of the gratings, the Bragg wavelength of the reflected light changes. Bragg sensor products are still in the early stages of development and use. The most obvious advantage of FBGs sensors is the high sensitivity. Such sensors can sense any change induced by pressure, strain, temperature, acceleration, and vibration. Ironically, this sensitivity in some applications becomes a disadvantage, because it is so sensitive that special care of the operating environments is needed to maintain a steady signal background. Furthermore, sensor instrumentation becomes very complicated and expensive since it must be capable of distinguishing the external change from among pressure, temperature, or other factors.
None of the conventional sensors are capable of providing the advantages of measuring extremely high pressure with tolerance of temperature extremes.